A wireless cell network, for instance the GSM network, includes a plurality of ground radio base stations which are connected to each other through the wired telephone network and which can be accessed by mobile terminals when the mobile terminals are situated within the radio cell of one station.
Each base station must have a sufficient range to assure that it overlaps into the neighboring cells in order to avoid any danger of the communication being cut when a mobile terminal changes cells. As a result the stations' transmitted powers must be larger than the required rigorous minimum.
Accordingly a mobile wireless terminal might receive at effective signal strength the transmissions from two stations when it is situated within an overlap zone of two cells. In practice the mobile wireless terminal receives many more signals and, if energized, links up with the nearest station, in the sense of radio transmission, namely the one which applies to it the highest radio electric field strength from the six optimal cells. As regards the overlap zone, the transmission from a station which is rejected by the terminal therefore constitutes an interference signal of the same order of magnitude as the useful signal from the hook-up cell. Moreover, because the station signals consist of time frames of the same structure, the terminal is unable to implement a specific spectrum filtration to attenuate the interfering signal(s).
In order to set up the wireless network, the operator can control a range of frequencies allocated to him and constituting a precious resource. The operator regularly distributes carrier frequencies, or transmission channels within the range. A frequency gap separating two carrier frequencies is large enough that inter-channel interference—in the light of the receiver selectivities—remains below a specific threshold of good operation.
However the number of available carrier frequencies is much lower than the number of channels required for the total network. Consequently the same frequency must be exploited in the network. However, each time the same frequency is used care must be taken that in each server cell the signal strength of the interfering signals at the frequency of its carrier(s) and incident from the stations of the other neighboring cells must remain below a sound operating threshold. In other words, at any point in the cell, there must be a safety margin between the received useful signal and the field strength of the interfering signal(s) at that frequency. However, calculating what these interferences are in a network including several thousand stations that interfere with one another requires a long computing times using an average power conventional calculator.
Considering only one cell in a conventional calculation, an estimate based on an algorithm is used for the various frequencies for their mutual interferences with the other cells within radio range, and the frequency corresponding to least interference is then selected. Next the algorithm considers a near cell and repeats those steps. In this manner, the frequencies are allocated stepwise throughout the network.
In this manner the various interferences received at the various frequencies constitute a matrix of the constraints relating to each cell and indicating the rejection weights of the various frequencies.
Accordingly this is an algorithm systematically selects the optimal local gradient within each cell of an interference function. The “output” variable of the matrix is a frequency having a value dependent on the “input” variable which is the interference.
When stated in conceptual form, a gravity-determined line of largest slope moves in a trough.
However this algorithm fails to be optimal: Even if—when selecting the cell frequencies—the interferences it generates are taken into account. Conventionally the initial selections by hypothesis in turn determines the selection of the downstream cells left unquestioned. This logic is set once and forever.
To return to the above previously mentioned concept, it is impossible to rise along the slopes of tile valley to check if an adjacent valley would serve better.
In other words, the algorithm is unidirectional whereas the interference constraints are “mutual,” that is bi-directional. The algorithm therefore poorly fits the problem and moreover its convergence time for scheduling frequencies is long because the mutual constraints entail a large number of radio stations which are in mutual radio range. The computational load substantially varies exponentially with the number of stations (the so-called full-NP problem).
An objective of the present invention is to at least reduce, all other things being equal, the convergence time of the algorithm.